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C₁₈ and other carbons

The cyclo[18]carbon (C₁₈) structure involves an alternating charged carbon structure in contrast to the uncharged structures commonly described.

 

Novel fulleranes

C₁₈
Novel carbons

C₁₈

The synthesis of one molecule of the sp-hybridized molecular carbon allotrope, cyclo[18]carbon (C₁₈), ^a has generated topical interest in its structure. It is generally accepted that C₁₈ forms a planar ring and may be either cumulenic with D9h (Figure 1a) or D18h symmetry (Figure 1b) with all bond lengths equal or polyynic with C9h symmetry (Figure 1c) with alternating different bond lengths, ^{c, d}. The polyynic form has been found in the newly synthesized C₁₈, using high-resolution atomic force microscopy.^a This agrees with theoretical approaches. ^f

 

Figure 1. Quantum chemistry calculated structures for cyclo[18]carbon; a, D9h cumulenic; b, D9h cumulenic;

c, C9h polyynic; d, the result of Restricted Hartree-Fock wave function (RHF) utilizing the 6-31G** basis set

 

 

The ground-state structures may also exhibit aromaticity with the carbon ring generating two sets of C_4N+2 electron rings in the plane of the ring and at right–angles to the plane ^{b, e}, as in Figure 1d. Whilst all the structures considered in the previous literature (Figure 1a, b, c) result in the carbon atoms being uncharged and equivalent, it has come as a surprise that if the structure is treated at the level of Restricted Hartree-Fock wave function (RHF) utilizing the 6-31G** basis set, and starting from a randomly placed ring structure, the carbon atoms are alternately positively charged (+0.38 e-) and negatively charged (-0.38 e-).The bond lengths alternate 1.3787 Å and 1.1978 and the bond angles alternate 163.3 ° (- + -) and 156.7 ° (+ - +). Clearly, the structure may converge to two possible forms, both where any pre-set carbon atom ends up as either positive or negative. Although this structure may be tconsidered as an intermediate form, it is a stable structure existing in a broad energy valley. These parameters (Figure 1d) should be compared with those for the DFT-B3LYP/6-31G* ^g as shown in Figure 1a, b, c. The bond angles in Figure 1d are identical to those given,^f using HF/6-31G*, for the polyacetylenie structure (156.7' and 163.3'). The 3-D isosurface of the total charge density is shown at the top right of the page, where it can be seen to be similar to that given in the AFM image in Figure 3 of ^a). This poly-charged structure would be liable to charged interactions with itself and other molecules in addition to the van der Waals forces previously considered.

 

In order to establish this structure for C₁₈, the structures of several cyclocarbons, C₆, C₁₀, C₁₄, C₁₈, C₂₂, and C₂₆, were calculated. The structures of C₆ and C₁₀ were more angular than the others, but still have equal bond lengrhs.

 

Cyclocarbon aromatic structures

Cyclocarbon	n	Diameter	r1 , Å	r2, Å	"+ - +", °	"- + -", °	C, alternating charge
C6	6	2.55 Å	1.3164	1.3164	149.57	90.43	+0.031	-0.031
C10	10	4.08 Å	1.2902	1.2902	119.35	168.65	+0.073	-0.073
C14	14	5.74 Å	1.2172	1.3591	139.8	168.8	+0.242	-0.242
C16	16	6.67 Å	1.1978	1.3828	152.2	162.8	+0.34(6)	-0.34(6)
C18	18	7.42 Å	1.1978	1.3787	156.7	163.3	+0.38(2)	-0.38(2)
C22	22	9.05 Å	1.196	1.3788	161.5	165.8	+0.44(4)	-0.44(4)
C26	26	10.68 Å	1.195	1.378	164.5	167.8	+0.49(6)	-0.49(6)

 

Cyclocarbon total charge densities

 

Rather counter-intuitively, belterene-type structures (for example, see (C₆H₂.C₂H₂)₄ below left) dissociate into two C₁₆ rings (see below right) if the hydrogen atoms are removed (by modeling using the Restricted Hartree-Fock wave function (RHF) utilizing the 6-31G** basis set)

 

Poly-arenes, e.g., (C₆H₂.C₂H₂)₄ dissociate, if the hydrogen atoms are removed

 

Novel carbons

Carbon-only structures with identical topologies as the oxygen atoms in the water ices are also possible. Thus, diamond (see right) has the same topology as cubic ice (and the tetrahedral water cluster as part of {(H₂O)₁₄}₂₀, consisting of 14 water molecules). Other carbons with topologies similar to the oxygen nets of crystalline ice phases or clathrates have been predicted, including some with a density greater than diamond that may exist at high pressure [2261]. Complex carbon nanostructures have been found in diamond. ^h

 

 

 

 

 

A new carbon biphenylene sheet has been discovered that is atomically flat and thin like graphene, but is made up of sp²-hybridized carbon atoms in ordered squares, hexagons, and octagons (see left ⁱ) with electronic properties very different (metallic) from those of graphene.

 

Another sheet structure is γ-graphyne; benzene rings joined hexagonally by carbon-carbon triple bonds from each carbon atom [4491].

 

 

 

 

 

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This page was established in 2019 and last updated by Martin Chaplin on 28 August, 2022